Dimensionless parameter method for evaluating decentralized water reuse systems in buildings

The performance of the decentralized water reuse systems that are widely implemented for the conservation of building water is affected by various characteristics, such as the location and type of building. Generalized methods to evaluate and compare different system configurations under various scenarios are currently lacking. Previously devised methods have focused on specific parameters describing buildings and are therefore not suitable for regionalized application. This study proposes a dimensionless parameter method for the evaluation of three decentralized systems in buildings with stable and seasonal daily non-potable water demands: rainwater harvesting systems (RWHs), graywater recycling systems (GWRs), and hybrid rainwater-graywater systems (HRGs). Japan was selected as a case study to illustrate the feasibility of this method. The results indicate that the favorable precipitation patterns in Japan support the use of RWHs rather than GWRs for conserving water, especially in buildings with seasonal daily non-potable water demands. Upgrading the existing systems to HRGs when RWHs and GWRs cannot meet the demand can increase the maximum water-saving efficiency by 40%. Thus, the method can effectively determine the optimum scenarios and configurations of RWHs, GWRs, and HRGs and provide policy guidance for the regional implementation of decentralized water reuse systems.